单选题
单选题
What is the class mainly discussing?
【正确答案】
D
【答案解析】[听力原文]
Narrator
Listen to part of a lecture in an ecology class.
Professor
So—uh, continuing our discussion of ecological systems, whole systems...the main thing to keep in mind here is the interrelationships. The species in a system, uh—and even the landscape itself—they"re interdependent. Let"s take what you read for this week, and see if we can"t apply this interdependence idea. Mike?
Male student
Well, um, how about beavers—ecosystems with beavers and waterways.
Professor
Good, good. Go on.
Male student
Like—well, you can see how it"s so important, "cause if you go back before Europeans settled in North America, like before the 1600s, back when Native Americans were the only people living here—well, back then there were a lot of beavers. But later on, offer Europeans—
Professor
OK, wait—I see where you"re heading with this, but before we go into how European settlement affected the ecosystem, tell me this: what kind of environment do beavers live in? Think about what it was like before the European settlers came. We"ll come back to where you were headed...
Female student
OK, well, beavers live near streams and rivers...And they block up the streams and rivers with, like, logs, and sticks, and mud, you know, they build dams, that really slow down the flow of the stream. So then the water becks up and creates, like, a pond that floods the nearby land.
Professor
And that creates wetlands. OK, tell me more.
Female student
Well, with wetlands it"s like there"s more standing water, more still water around. And that water"s a lot cleaner than swiftly flowing water because the dirt and sediment and stuff has a chance to sink to the bottom.
Professor
More important for our discussion: Wetland areas support a lot more varieties of life than swiftly flowing water. For example, there are more varieties of fish, of insects, lots of frog species. And then species that rely on those species start to live near the wetlands, too.
Female student
Yes. Like birds and mammals that eat the fish and insects. And you get trees and plants that begin to grow near the standing water that can"t grow near running water. Oh, and there"s something about wetlands and groundwater, too...
Professor
OK. Good. Wetlands have a big effect on groundwater—the amount of water below the surface of the land. Think of wetlands as...um, like a giant sponge. The earth soaks up a lot of this water that"s continually flooding the surface, which increases the amount of water below. So, where there are wetlands, you get a lot of groundwater. And groundwater happens to be a big source of our own drinking water today. All right. So back to the beavers. What if the beavers weren"t there?
Male student
You"d just have a regular running stream, "cause there"s no dam. So the ecosystem would be completely different—there"d be fewer wetlands.
Professor
Exactly. So, now let"s go back where you were headed before, Mike. Uh, you mentioned a change that occurred after Europeans came to North America...
Male student
Yeah—well, there used to be beavers all over the place...um, something like 200 million beavers, just in the continental United States. But when Europeans came, they started hunting the beavers for their fur..."cause beaver fur, it"s really warm—and it was really popular for making hats in Europe. So the beavers were hunted a lot, overhunted—they were almost extinct by the 1800s. So...that meant fewer wetlands—less standing water.
Professor
And what does that mean for the ecosystem? Kate.
Female student
Well, if there"s less standing water, then the ecosystem can"t support as many species, because a lot of insects and fish and frogs can"t live in running water. And then the birds and animals that eat them lose their food supply.
Professor
Precisely. So the beaver, in this ecosystem, is what we call a keystone species.
Narrator
Listen to part of a lecture in an ecology class.
Professor
So—uh, continuing our discussion of ecological systems, whole systems...the main thing to keep in mind here is the interrelationships. The species in a system, uh—and even the landscape itself—they"re interdependent. Let"s take what you read for this week, and see if we can"t apply this interdependence idea. Mike?
Male student
Well, um, how about beavers—ecosystems with beavers and waterways.
Professor
Good, good. Go on.
Male student
Like—well, you can see how it"s so important, "cause if you go back before Europeans settled in North America, like before the 1600s, back when Native Americans were the only people living here—well, back then there were a lot of beavers. But later on, offer Europeans—
Professor
OK, wait—I see where you"re heading with this, but before we go into how European settlement affected the ecosystem, tell me this: what kind of environment do beavers live in? Think about what it was like before the European settlers came. We"ll come back to where you were headed...
Female student
OK, well, beavers live near streams and rivers...And they block up the streams and rivers with, like, logs, and sticks, and mud, you know, they build dams, that really slow down the flow of the stream. So then the water becks up and creates, like, a pond that floods the nearby land.
Professor
And that creates wetlands. OK, tell me more.
Female student
Well, with wetlands it"s like there"s more standing water, more still water around. And that water"s a lot cleaner than swiftly flowing water because the dirt and sediment and stuff has a chance to sink to the bottom.
Professor
More important for our discussion: Wetland areas support a lot more varieties of life than swiftly flowing water. For example, there are more varieties of fish, of insects, lots of frog species. And then species that rely on those species start to live near the wetlands, too.
Female student
Yes. Like birds and mammals that eat the fish and insects. And you get trees and plants that begin to grow near the standing water that can"t grow near running water. Oh, and there"s something about wetlands and groundwater, too...
Professor
OK. Good. Wetlands have a big effect on groundwater—the amount of water below the surface of the land. Think of wetlands as...um, like a giant sponge. The earth soaks up a lot of this water that"s continually flooding the surface, which increases the amount of water below. So, where there are wetlands, you get a lot of groundwater. And groundwater happens to be a big source of our own drinking water today. All right. So back to the beavers. What if the beavers weren"t there?
Male student
You"d just have a regular running stream, "cause there"s no dam. So the ecosystem would be completely different—there"d be fewer wetlands.
Professor
Exactly. So, now let"s go back where you were headed before, Mike. Uh, you mentioned a change that occurred after Europeans came to North America...
Male student
Yeah—well, there used to be beavers all over the place...um, something like 200 million beavers, just in the continental United States. But when Europeans came, they started hunting the beavers for their fur..."cause beaver fur, it"s really warm—and it was really popular for making hats in Europe. So the beavers were hunted a lot, overhunted—they were almost extinct by the 1800s. So...that meant fewer wetlands—less standing water.
Professor
And what does that mean for the ecosystem? Kate.
Female student
Well, if there"s less standing water, then the ecosystem can"t support as many species, because a lot of insects and fish and frogs can"t live in running water. And then the birds and animals that eat them lose their food supply.
Professor
Precisely. So the beaver, in this ecosystem, is what we call a keystone species.
单选题
Why does the professor interrupt the student when he first mentions European settlement in North America?
【正确答案】
C
【答案解析】
单选题
What does the professor say about still water and swiftly flowing water?
【正确答案】
C
【答案解析】
单选题
According to the professor, what was the impact of the extensive hunting of beavers in North America?
【正确答案】
A
【答案解析】